Washover events on coastal headland beaches occur when storm surge from cold fronts, high tides, tropical storms, and hurricanes move across the beach, reworking and depositing sand in the back marshes and shallow mudflat areas. On Fourchon Beach, Louisiana, a 9-mile coastal headland beach impacted by the Deepwater Horizon oil spill, these washover events have intermittently moved oil from the subtidal and intertidal portion of the beach to the supratidal mudflats and marsh areas. In order to determine the impacts of washover events on oil fate, 4 complementary studies are being undertaken. First, washover areas (N=56) have been mapped and sampled for PAHs and n-alkanes in the most heavily impacted area eastern end of Fourchon Beach. Samples indicate the most heavily impacted segments were in areas where a physical breach of the beach developed. Second, characteristics of specific washover events and locations are being studied to establish typical categories of events (i.e., breaches, remobilization of oil on the beach and mobilization of subtidal oil mats) as well as the characteristics of oil mobilized by each category. As an example, presence of MC252 oil was mapped using soil cores in a washover mudflat behind a breach, which developed during TS Isaac in 2011. MC252 oil was present at an average of 6 – 12 cm below the surface. Concentrations of PAHs in this layer ranged from 1.2 to 16 mg/kg consisting primarily of C1-C4 phenanthrenes, C2 and C3 dibenzothiophenes and C1-C2 chrysenes. Third, the chemical signature of crude oil including a range of petrogenic PAHs and n-alkanes relative to poorly biodegradable C30-hopane is being used at these locations to assess the relative weathering of oil at various stages in the washover process. Finally, a laboratory microcosm study is being conducted to understand the biogeochemical controls on oil biodegradation in the new environment (washover mudflat) and how these controls impact its ability to degrade naturally. Anaerobic treatments consistent with the flooded mudflat areas are amended with N and sulfate in various combinations to determine if nutrients or electron acceptors are limiting in the washover environment. Results to date indicate that most PAHs are stable under the flooded conditions with the exception of C1-phenanthrenes. These studies will be discussed in the context of remedial measures during the emergency response phase used on Fourchon Beach to prevent washover impacts.

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